Magnetic oxide nanocrystals of the first row transition metals, e.g., Cr, Mn, Fe, Co, and Ni are important for an understanding of the magnetic properties in the nanometer regime, and several technical applications ranging from magnetic resonance imaging, drug delivery, battery materials, catalysts, biosensing, nanoelectronic materials, etc. Realization of these goals relies on the availability of size- and shape-controlled nanocrystals. Previously, there is no general method reported for the synthesis of monodisperse magnetic oxide nanocrystals with size- and shape-control.
Colloidal magnetic oxide nanocrystals are traditionally synthesized through the precipitation of nanocrystals from basic aqueous solutions with a broad size distribution. [see, e.g., Vestal, C., et al., JACS, 2002, 124, 14312-14313, and references therein] Synthesis of oxide nanocrystals has recently been directed to non-aqueous approaches. [Trentler, T., et al., JACS,. 1999, 121, 1613-1614; Rockenberger, J., et al., JACS, 1999, 121, 11595-11596; O'Brien, S., et al., JACS, 2001, 123, 12085-12086; Hyeon, T., et al., JACS, 2001, 123, 12798-12801; Sun, S., et al., JACS, 2002, 124, 8204-8205; Pacholski, C., et al., Angew. Chemie 2002, 41, 1188-1191; Urban, J., et al., JACS, 2002, 124, 1186-1187; Seo, W., et al., Adv. Mater. 2003, 15, 795-797; Yin, M., et al., JACS, 2003, 125, 10180-10181; Lee, K., et al., JACS, 2003, 125, 3408-3409; Monge, M., et al., Angew. Chem., 2003, 42, 5321-5324], mostly inspired by the synthesis of high quality semiconductor nanocrystals in non-aqueous media. [Murray, C., et al., JACS, 1993, 115, 8706-8715; Peng, X., et al., JACS, 1998, 120, 5343-5344; Peng, Z., et al., JACS, 2001, 123, 183-184]
The quality of the nanocrystals yielded by non-aqueous solution methods is generally better than that of the nanocrystals synthesized in aqueous solutions. Hyeon et al., supra, reported that γ-Fe2O3 nanocrystals were synthesized using an organometallic compound, Fe(CO)5, as the precursor and trimethylamine oxide as an oxidant in a non-aqueous solution. Recently, Sun et al., supra, demonstrated the formation of nearly monodisperse Fe3O4, CoFe2O4, and MnFe2O4 nanocrystals [Sun, S., et al., JACS, 2004, 126, 273-279] using metal acetylacetonates as the precursor in the presence of 1,2-hexadecanediol, oleylamine, and oleic acid in phenol ether. Yin et al., supra, reported that relatively monodisperse MnO nanocrystals were formed using manganese acetate as the precursor in a coordinating solvent composed of oleic acid and trioctylamine. The sizes of the resulting nanocrystals in these high temperature and non-aqueous solution approaches varied between a few nanometers and about 20 nm, mostly by secondary injections of the precursors—seeded growth—and Ostwald ripening. The size distribution of the magnetic oxides was reported in these three papers, however, shape control of the nanocrystals was seldomly discussed. Very recently, Cheon et al. reported that, without using trimethylamine oxide as oxidant, the organometallic approach yielded γ-Fe2O3 nanocrystals composed of a mixture of several different shapes. [Cheon, J., et al., JACS, 2004, 126, 1950-1951]
The diverse structures and properties of metal oxides make it difficult to develop a general synthestic strategy for these nanocrystals. Greener synthetic approaches for II-VI and III-V semiconductor nanocrystals have been developed almost exclusively based on metal carboxylate salts, which are compatible with non-aqueous media. [Peng, X., Chem. Eu. J. 2002, 8, 334-339] It is known that metal carboxylates decompose at elevated temperatures and metal oxides are common decomposition products in many cases. Colloidal II-VI semiconductor nanocrystals are the most developed ones in terms of synthetic chemistry due to the success of organometallic approaches and the alternative (or greener) approaches. [Peng, X., et al., supra; Qu, L., et al., Nano Lett. 2001, 1, 333-336; Yu, W., et al., Angew. Chemie Int. Ed. 2002, 41, 2368-2371]. The key to this success, as revealed by the mechanism studies, is to maintain a balance between the nucleation and growth stages. This balance can be better achieved by non-coordinating solvent approaches introduced recently. This may be because the reactivity of precursors in non-coordinating solvents can be fine-tuned by varying the bonding strength of the ligands to the monomers, the concentration, chain length, and/or configuration of the ligands for the monomers. [Yu, W., et al., supra; Yu, W., et al., Chem. Mater. 2003, 15, 4300-4308; Battaglia, D., et al., Nano Lett. 2002, 2, 1027-1030]
Synthesis of ZnO nanocrystals and other types of nanostructures has attracted significant attention in recent years because of its potential as UV emitting materials, catalysts, host materials for doped nanocrystals, etc. Transparent conducting oxides (TCOs) have found a variety of applications due to their best available performance in terms of conductivity, transmissivity, excellent stability and good surface morphology. Among them, indium oxide, a wide band gap n-type semiconductor, and indium tin oxide (ITO) have been employed as microelectronic device materials in solar cells, flat panel displays, sensors and architectural glasses. Also, sensing devices based on metal oxide semiconductors like In2O3 and tin oxide (SnO2) are used for the detection of carbon monoxide and nitrogen oxides. Materials with high surface area are advantageous for obtaining a good sensitivity in sensing applications. It has been demonstrated that a decrease in the size of the metal oxide crystallite leads to a considerable increase in sensitivity. [Yamazoe, N. Sens. Actuators B. 1991, 5, 7] A few reports on the preparation of In2O3 nanoparticles and nanowires have appeared recently in the literature. [Zhou, H., et al., Appl. Phys. Lett. 1999, 75, 495; Murali, A., et al., Nano Lett. 2001, 1, 287; Liu, Q., et al., JACS, 2005, 127, 5276] Most reports on the synthesis of nanostructured SnO2 have focused on the direct oxidation of metallic tin, tin(II) halides etc. [Pan, Z., et al., Science 2001, 291, 1947; Dai, Z., et al., J. Phys. Chem. B 2002, 106, 1274; Chen, Y., et al., Chem. Phys. Lett. 2003, 369, 16; Deng, H., et al., Chem. Mater. 2003, 15, 2429]
Shape control is important for nanocrystals because of their morphology-dependent fundamental and technical importance. A model has been established for shape and size control of the mostly studied II-VI semiconductor nanocrystals, which implies non-equilibrium shaped nanocrystals require high remaining monomer concentrations—low yield reactions—to prevent Ostwald ripening and intra-particle ripening. [Peng, X., et al., Nature (London) 2000, 404, 59-61; Peng, Z., et al., JACS, 2001, 123, 1389-1395; Peng, Z., et al., JACS, 2002, 124, 3343-3353; Lee, S.-M., et al., Advanced Materials (Weinheim, Germany) 2003, 15, 441-444; Manna, L., et al., Nature Materials 2003, 2, 382-385] Similarly, focusing of size distribution, which is the process needed for the growth of monodisperse dot-shaped nanocrystals, occurs only when monomer concentration is higher than the solubility of all nanocrystals in the solution. When the monomers deplete to a certain level, defocusing of size distribution (Ostwald ripening) occurs and a broad size distribution will be the result. These established models seem to imply that monodisperse high quality nanocrystals, especially when non-equilibrium shaped, cannot be obtained without some monomers remaining in the solution.
Shape-controlled growth of crystals in solution has traditionally been called crystal habits and has been explained by two models, Wuff facet theory and surface additive mediated growth, which are also applied for explaining the growth of colloidal nanocrystals and nanostructures. Studies of nanocrystal growth have revealed several new routes, such as, template-directed, oriented attachment, photoradiation-induced growth, and monomer activity mediated growth.
In the patent literature, U.S. Pat. No. 6,225,198 (issued to Alivisatos et al.) proposes a method for controlling the shape of semiconductor nanocrystals by adjusting the ratio of surfactants in a mixture of Group II and VI precursors. U.S. Pat. No. 6,440,213 (issued to Alivisatos et al.) proposes a method of making surfactant-capped nanocrystals of transition metal oxides. U.S. Pat. No. 6,855,202 (issued to Alivisatos et al.) proposes a method of making shaped nanocrystal particles comprising a plurality of crystal structures. U.S. Pat. No. 6,872,249 (issued to Peng et al.) proposes a method of synthesizing nearly monodisperse Cd chalcogenide nanocrystals. U.S. Patent Publication 2004/0101976 (of Peng et al.) reports a method of stabilizing colloidal suspensions of nanocrystals by coating the crystals with bulky organic dendron molecules. U.S. Patent Publication 2005/0129947 (of Peng et al.) reports a method of making nearly monodisperse colloidal semiconductor nanocrystals having a core/shell structure. None of the above references propose a general method for making high quality metal oxide nanocrystals having a monodisperse size distribution and/or a controlled shape.